\begin{tikzpicture}
    \coordinate [label=180:$a_2$] (a2) at (0, 0);
    \coordinate [label=180:$a_1$] (a1) at (0, 2);
    \coordinate [label=0:$b_1$] (b1) at (2, 2);
    \coordinate [label=0:$b_2$] (b2) at (2, 0);

    \draw (a1) -- (b2);
    \draw [dashed] (a2) -- (b1);
\end{tikzpicture}
